An improvement of the tire's characteristics such as low noise and wet resistance characteristics as well as the riding comfort and steering stability are demanded. And it is known that a tread pattern is one of the factors that affect such characteristics.
There are such tread patterns as the lug pattern mainly composed of lateral grooves, the rib pattern mainly composed of circumferential grooves, the rib and lug pattern which intermediates the former two and the block pattern having blocks divided by circumferential and lateral grooves. And it is known that although, specifically, a tire having the block pattern is generally superior in characteristics against the road surface such as driving performance and hydroplaning characteristic, it is generally inferior in the steering stability concerned with the cornering power and wear resistance due to the relatively low rigidity of the blocks. However, in radial tires which are widely used in these days, the rigidity of the tread part is increased by a belt layer having superior hoop effect, and the wear resistance and steering stability are improved by employing harder tread rubbers. As the result, tires with the block pattern are being used in high-speed buses and passenger cars.
Therefore, as the application range of a tire is widened, it is more greatly required that the characteristics of a tire having such tread pattern are improved.
On the other hand, the circumferential and lateral grooves are factors that affect the tire's characteristics, and it is known that, when the total groove area of the circumferential and lateral grooves in the ground contact surface is smaller, the wet grip performance is reduced, while the low noise characteristic that relates to pattern noises is improved.
FIG. 2, for example, shows a result of measuring the pattern noise by changing the total groove area S.sub.T, which is the sum of the circumferential and lateral groove areas S.sub.G and Sg, and aligning the grooves in the same manner. The total groove area quotient S.sub.T /S in the figure is the quotient of the total groove area S.sub.T in the ground contact surface to the ground contact area S which is the area of the ground contact surface, and a pattern noise (dB) is shown on the axis of ordinates.
As shown in FIG. 2, as the total groove area quotient S.sub.T /S increases, and thereby the groove area is increased, it is recognized that the pattern noise is increased. However, the groove area is an inverse factor, as described hereinbefore, in regard to the wet resistance and pattern noise characteristics. Therefore, it is required to improve both characteristics of a tire in good balance.
The invention has been achieved as a result of various studies on the areas of circumferential and lateral grooves as well as their numbers. It is a primary object of the invention to present a pneumatic tire that can improve various characteristics in good balance in such manner that the wet resistance characteristic is improved, while reducing the pattern noise, and the riding comfort is also improved.
According to one aspect of the present invention, a pneumatic tire has a tread pattern divided by two circumferential grooves extending in the circumferential direction of a tire and eighty-five or more lateral grooves crossing the circumferential grooves in a tread part, wherein the quotient S.sub.T /S of the total groove area S.sub.T which is the sum of the circumferential groove area S.sub.G and the lateral groove area Sg in a ground contact surface when the tire is mounted on a standard rim, inflated with a normal internal pressure and loaded with a normal load to the ground contact area S is in a range from 0.33 to 0.53, and the quotient Sg/S.sub.G of the lateral groove area Sg to the circumferential groove area S.sub.G is in a range from 0.33 to 0.25.
In order to lower the pattern noise and maintain the low noise level, as shown in FIG. 2, in the first place, the total groove area quotient S.sub.t /S is defined in a range of 0.15 to 0.25. If the area quotient S.sub.t /S exceeds 0.25, the pattern noise is increased, and also as the groove area increases, the block area is decreased, which may give rise to the shortage of dry grip strength and cornering power, sacrifice of steering stability and drop of wear resistance. Meanwhile, if the quotient S.sub.t /S is less than 0.15, the wet resistance is significantly lowered, and the riding comfort and other tire performance on the premise of the existence of grooves cannot be exhibited.
By setting the area quotient S.sub.t /S in a range of 0.25 or less, as mentioned above, the wet resistance performance is generally lowered. Therefore, various studies were prosecuted in order to enhance the wet resistance as far as possible even if the quotient S.sub.t /S is 0.25 or less. As the result, the present inventors took notice of the difference of the degree of effect of the circumferential grooves and lateral grooves on the wet resistance characteristic, and continued discussions and studies, mainly using tires in the size of 195/60R15, concerning the optimum selection about the distribution rate of the circumferential groove area S.sub.G and lateral groove area Sg, that is, the area quotient of lateral grooves and circumferential grooves Sg/S.sub.G, as the quotient of the lateral groove area Sg to the circumferential groove area S.sub.G.
Accordingly, in order to see the relationship between the hydroplaning onset velocity which are important factors among the wet resistance characteristics, circumferential groove area quotient (S.sub.G /S), the quotient of the circumferential groove area S.sub.G in a ground contact area S divided by the ground contact area S, and lateral groove area quotient (Sg/S), the quotient of the lateral groove area Sg in a ground contact area S divided by the ground contact area S, the hydroplaning onset velocity was measured by using tires with tread patterns P1 to P5 with 1 to 4 circumferential grooves as shown in FIG. 5 to FIG. 9. The results are shown in Table 1.
On the basis of the results disclosed in Table 1, and also adding other measurement date, the following formula was obtained through a multiple regression analysis with the circumferential groove area quotient S.sub.G /S and lateral groove area quotient Sg/S as independent variables, and the hydroplaning onset velocity (V: km/h) as a dependent variable: EQU V=74.5.times.S.sub.G /S+31.0.times.S.sub.g /S+61.7
where S is the ground contact area as mentioned hereinbefore.
From the formula, it is recognized that the contribution of the lateral groove area quotient Sg/S to the hydroplaning onset velocity V is 31.0/74.5 times, or approximately 0.42 times, the circumferential groove area quotient S.sub.G /S. Therefore, when the total groove area quotient S.sub.t /S is regulated, it is evident that the increase of the circumferential groove area quotient S.sub.G /S is effective only in regard to the hydroplaning onset velocity V, and further improvement can be achieved by eliminating the lateral grooves g to only leave the circumferential grooves G. In regard to only the other characteristics, specifically the riding comfort which is related to the envelope power, however, it is known that the lateral grooves g are indispensable. But it is shown by the formula that if the circumferential grooves G are eliminated and only the lateral grooves g are formed in order to improve the riding comfort, the wet resistance characteristic is deteriorated.
Therefore, an optimum balancing between the circumferential groove area quotient S.sub.G /S and the lateral groove area quotient Sg/S in the total groove area quotient S.sub.t /S is essential for the tire performance.
The inventors found that it is preferred to select the area quotient Sg/S.sub.G of the lateral grooves to the circumferential grooves according to the contribution ratio of the circumferential groove area quotient S.sub.G /S and the lateral groove area quotient Sg/S to the hydroplaning onset velocity, that is between 0.33 and 0.53 centered by the quotient 0.42.
That is, supposing that both circumferential and lateral grooves are indispensable, it is not an appropriate method to increase the lateral groove area Sg which is less contributory to the wet resistance characteristic that is an important characteristic of a tire, and therefore the tire's characteristics can be improved in good balance while preventing the deterioration in the wet resistance characteristics by forming the circumferential and lateral grooves in such manner that the contribution of the lateral grooves area quotient Sg/S is approximately 0.42 times (0.3 to 0.53 times) the circumferential groove area quotient S.sub.G /S.
In order to improve the tire's characteristics even in such conditions that the total groove area quotient S.sub.t /S is in a range from 0.15 to 0.25, and the area quotient Sg/S.sub.G of the lateral grooves to the circumferential grooves is in a range from 0.33 to 0.5, in practically selecting a tread pattern, it comes to be indispensable to choose the optimum number of the circumferential and lateral grooves.
FIG. 3 shows a measurement result of the hydroplaning onset speed by changing the number of circumferential grooves, in which the number of circumferential grooves is shown on the axis of abscissas and the hydroplaning onset speed on the axis of ordinates by indices. In the measurement shown in FIG. 3, the influences from the groove area were eliminated by changing the circumferential groove width while maintaining the total groove area S.sub.T to be a specified area. As evident in FIG. 3, it is recognized that the hydroplaning onset speed is reduced and the wet resistance characteristic is deteriorated as the number of circumferential grooves is increased. Thus, when the total groove area is set at a specified value, it is found that the wet resistance characteristic is improved by reducing the number of circumferential grooves and increasing the area of each circumferential groove.
As a result of what is described hereinabove, the wet resistance characteristic comes to be improved by forming two or less circumferential grooves. However, in the case of one circumferential groove, a wider circumferential groove should be formed on the tire's equator. As the result, the steering stability and resistance to uneven wear are deteriorated, whereby the optimum number of circumferential grooves is two.
FIG. 4 shows a measurement result of the pattern noise and reactive force in running over a bump in a tire with two circumferential grooves formed in its tread pattern by maintaining the total groove area S.sub.T at a specified value and changing the number of lateral grooves. The number of lateral grooves is shown in the axis of abscissas, and the pattern noise and reactive force (in indices) are shown on the axis of ordinates. It is known from FIG. 4 that, when there are many lateral grooves, especially, the number thereof is 85 or more, as the area of each lateral groove can be reduced, the pattern noise is lowered so as to improve the low noise characteristic. And, as the reactive force in running over a bump is reduced, the enveloping power is increased to improve the riding comfort as well, thus properly improving the tire's characteristics in good balance.